1. Field of the Invention
The present invention relates to an image heating device employing an induction heating system, which heats a toner image on a recording material. The image heating device may be used in an image forming apparatus such as a copying machine, a printer, a facsimile machine, and a multifunctional peripheral having multiple functions thereof.
2. Description of the Related Art
Hitherto, in an electrophotographic fixing device (image heating device), fixing processing (heating processing) has been performed by pressurizing and heating a toner image formed on a recording material at a nip portion between a fixing roller (heating rotary member) and a pressure roller.
As measures that respond to the demand for energy saving of recent years, there has been proposed a fixing device employing an induction heating system that uses high-frequency induction as a heating source (Japanese Patent Application Laid-Open No. 2010-160388). This induction heating device includes a magnetic flux generating unit including an exciting coil and a magnetic core. With the high-frequency magnetic field generated by causing a high-frequency current to flow through the exciting coil, an induction eddy current is generated in the fixing roller (heating rotary member), and thus the fixing roller itself generates Joule heat by its skin resistance. When a leading end portion of the magnetic core opposed to the fixing roller has a large thickness in its circumferential direction, it is difficult to increase the maximum magnetic flux density of the magnetic flux acting to the fixing roller. As a result, the time rate of change of the magnetic flux acting to the fixing roller does not increase, and hence the heat generation efficiency may be reduced. When the core opposed to the fixing roller is thinned from its root, it becomes difficult to maintain the strength of the core itself.